Sandgren's parents, South African Lia Lourens and American David Sandgren, met at a tennis club in Johannesburg. They married and moved to Tennessee in 1988 with Sandgren's older brother, Davey.

Sandgren, who was named Tennys after his Swedish great-grandfather, was home-schooled and is coached by his mother. Davey Sandgren is also a tennis player who achieved a career-high ATP ranking of 800 in doubles in 2009.

Sandgren was a January midseason addition to the Tennessee Volunteers' 2010 tennis roster, joining older brother, Davey, who was a senior All-American on the team.[4] Coached by Sam Winterbotham and Chris Woodruff, the younger Sandgren immediately strengthened the middle of the Vols' singles lineup, going 10-0 in Southeastern Conference play at the No. 4 position to help the team win the SEC regular-season and tournament titles.[5] The team reached the NCAA team finals that year.

As a sophomore, Sandgren reached the semifinals of the NCAA Singles Championships, losing to teammate, roommate and doubles partner Rhyne Williams in three sets. He finished the season with a 37-6 record and his 10-1 record in conference helped the Vols to an SEC regular-season title. He finished his career with a 60-12 singles record (83.33 percent), the third-best career winning percentage in Tennessee history.[6]

He was also a member of the USTA Summer Collegiate Team after his freshman and sophomore seasons.

2011–2016: Early years, Futures success and first Challenger titleEdit

Sandgren turned professional in 2011, playing mostly at the ITF Futures level through 2012. In 2013, his ATP ranking remained inside the top 300 for nearly the entire year, allowing him to play on the Challenger Tour for much of the season. In his final tournament of the year, Sandgren won his first final at Challenger level in Champaign to crack the Top 200 for the first time.

In 2014, Sandgren missed half of the year after undergoing hip surgery near the beginning of the season.[7] Upon returning to the court, it took him nearly a year to get back to the Top 250 and the Challenger level. Sandgren returned to the Top 200 in November 2016, when he reached his second career Challenger final in Columbus, almost three years after he first achieved these two milestones.

After having reached the quarterfinals at the Geneva Open, Sandgren was knocked out in the first round of the French Open by world No. 177 Hubert Hurkacz. He lost in the first round of the doubles tournament in straight sets.

In the first round of Wimbledon, Sandgren was knocked out by eventual champion Novak Djokovic in straight sets, winning only six games.

In the first round of the U.S. Open, Sandgren beat Viktor Troicki in straight sets before losing to Djokovic in four sets in the second round.

In January, Sandgren won his maiden ATP Tour title at the Auckland Open.[14] He achieved his career high in rankings at 41 on January 14. However, after the title, Sandgren suffered a 9-match tour-level losing streak, which he finally snapped at Wimbledon, where he reached the fourth round before losing to Sam Querrey. He then reached the quarterfinals at Newport and the third round at the U.S. Open. At the end of the season, a fractured toe kept him inactive, and his ranking sank back out of the top 100.

When questioned on his social media activity, Sandgren said he found some of the alt-right content interesting, but that to fully support alt-right beliefs would be incompatible with his Christian faith.[16][17][18][19] He later clarified: "Definitely not all 'alt-right' content is interesting, just some individuals' specific content".

Within 48 hours of the controversy emerging, Sandgren had deleted several years of tweets, claiming he wanted to create a "version of a cleaner start".[20]